Sheets or films made of crystalline thermoplastic resins, especially polypropylene resins, have different physical properties such as transparency, depending on the crystalline morphology thereof. Heretofore, the use of polypropylene resins has been limited though the resins have excellent physical properties such as heat resistance, moisture-proofness and oil resistance and they are lightweight and cause no environmental pollution. This is because when the resins are shaped into sheets or further into containers from the shaped sheets, the sheets and the containers have poor transparency.
In order to make polypropylene resin sheets transparent, it is necessary to make both surfaces of the sheet glossy and additionally to make the inside of the sheet transparent. To make both surfaces of the sheet glossy, for example, there is known a method of transferring a roll mirror face to them, a method of transferring a metal mirror face to them using an endless belt having the face, a method of increasing the surface gloss of a melt web extruded from a die, etc. On the other hand, in order to make the inside of the sheet transparent, for example, there are known (a) a method of inhibiting crystallization of the sheet or letting the sheet have fine crystals (having a particle size smaller than the minimum wavelength of visible rays) by modifying the raw materials, (b) a method of inhibiting crystallization of the sheet by rapidly cooling it, (c) a method of breaking the crystals in the sheet by stretching or orientation, etc.
Accordingly, various methods comprising of these means as combined have heretofore been proposed. For instance, there has been proposed a method of transferring a roll mirror face to the surfaces of a resin sheet while a nucleating agent is added to the raw resin, whereby both surfaces of the resin sheet are made glossy while the inside of the sheet is made transparent. According to the method, however, it is difficult to form a thin sheet and the transparency of the sheet formed is not complete.
Apart from the above, another method has also been proposed, in which a roll mirror face is transferred to both surfaces of a resin sheet while the sheet is rolled under pressure. According to the method, however, since the sheet is stretched and oriented by rolling under pressure, there occur various problems that the physical properties of the sheet in the MD direction and the TD direction become unbalanced and the thermal shapability of the sheet becomes poor.
In addition, transferring methods using a mirror face-having an endless metal belt have been disclosed in, for example, JP-B 61-24977, 61-32572, JP-A 62-284719. (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.) However, the transparency of the sheets to be produced by such methods is still unsatisfactory.
On the ether hand, the present applicant has heretofore proposed various techniques for controlling crystallization of polypropylene resin sheets or films by rapidly cooling them, as the means of improving their transparency without stretching the sheets or films (for example, JP-B 62-41457, 62-10816, JP-A 62-42822, 60-236719) and has already put them into practical use.
Recently, further improvement in the quality of sheet products and also in the their productivity has been desired. Given the situation, however, the conventional methods have various problems. For example, if the production speed is further elevated, the shear stress at the extrusion die increases to form a so-called shark skin on the surface of the sheet formed whereby the surface gloss of the sheet is lowered or, that is, the transparency thereof is lowered. In addition, if the thickness of the sheet to be formed is 0.6 mm or more, the transparency thereof is rapidly lowered due to the limit of the thermal conductivity in the direction of the thickness of the sheet. Accordingly, in view of the object of improving the productivity, a method of producing resin sheets at a higher speed is desired. In addition, also desired is a method of producing thick and highly-transparent resin sheets.
For these, the present applicant has already proposed a method for improving the transparency of a resin sheet, using an endless metal belt that was mirror-finished to have a surface roughness of 1.5 .mu.m or less for treating a resin sheet, in which a particular resin material composition is used so as to inhibit the crystallization of the sheet to be formed therefrom while the both surfaces of the sheet formed are made glossy with the belt, as the method for further improving the prior art techniques (Japanese Patent Application No. 4-27485).
However, the method still had various problems. When the temperatures of the roll and the belt used in the method are low, the sheet formed is often banked so that the outward appearance of the sheet becomes bad, rapid cooling of the sheet formed is impossible, and much time is needed for producing a thin sheet.